This invention relates to an adaptive device for separating a luminance signal and a color signal in color video systems or color television receivers. Such a device is generally called a Y-signal/C-signal separation filter.
In recent color television receives, a fixed-type two-dimensional filter using a comb filter is generally used to separately derive a luminance signal (a Y signal) and a color signal (a carrier chrominance signal, a C signal) from a composite color television signal.
Japanese published unexamined patent application 60-165889 discloses an adaptive comb filter for separating a luminance signal and a color signal. In the adaptive comb filter of Japanese application 60-165889, characteristics of a two-dimensional filter element are changed in accordance with conditions of adjacent picture elements.
The adaptive comb filter of Japanese application 60-165889 includes two different filter elements, that is, a vertical direction separation filter and a horizontal direction separation filter, each separately deriving a luminance signal and a color signal from a digitized composite color television signal. In the case where a given area surrounding a picture element has a smaller data variation in a vertical direction than a data variation in a horizontal direction, a luminance signal and a color signal outputted from the vertical direction separation filter are selected. In the opposite case, a luminance signal and a color signal outputted from the horizontal direction separation filter are selected. This selective change of the filter characteristics enables accurate separation of the luminance signal and the color signal in abruptly-varying portions of a picture, so that dot interference and cross color can be decreased.
In the adaptive comb filter of Japanese application 60-165889, since the filter characteristics are changeable between only two states and these states are extremely different, a luminance signal and a color signal tend to be considerably discontinuous in waveform and be thus contaminated by noises at moments corresponding to changes of the filter characteristics. In regions where the degrees of variations in the vertical and horizontal directions are similar so that the filter characteristics are frequently changed, such noises are conspicuous.
In the adaptive comb filter of Japanese application 60-165889, when the vertical direction separation filter is selected, the band of the luminance signal in the vertical direction is limited regardless of that in the horizontal direction. When the horizontal direction separation filter is selected, the band of the luminance signal in the horizontal direction is limited regardless of that in the vertical direction. Accordingly, the band of the luminance signal is sometimes decreased to a region without the color signal so that the resolution tends to be lowered.
In regions which have considerable variations in both of the vertical and horizontal directions, the adaptive comb filter of Japanese application 60-165889 causes the band of the color signal in one of the two directions to be widened so that cross color tends to be generated at an unacceptable level.
U.S. Pat. No. 4,907,073 (Japanese patent application 62-320965) discloses an adaptive device for separating a luminance signal and a color signal which is designed to resolve the previously-mentioned problems in the adaptive comb filter of Japanese application 60-165889. In the adaptive device of U.S. Pat. No. 4,907,073, a combination of a variable vertical-direction band pass filter and a variable horizontal-direction band pass filter extracts a color signal from a composite television signal. In addition, a vertical-direction variation detector detects a difference between levels of the composite television signal at sampling points separated vertically in a picture. A horizontal-direction variation detector detects a difference between levels of the composite television signal at sampling points separated horizontally in a picture. Pass bands of the vertical-direction band pass filter and the horizontal-direction band pass filter are controlled in accordance with output signals from the vertical-direction variation detector and the horizontal-direction variation detector. A luminance signal is derived from the composite television signal and the extracted color signal.
The adaptive device of U.S. Pat. No. 4,907,073 tends to cause crosstalk such as dot interference and cross color in the case of a television signal having a large amount of components corresponding to inclined spatial edges.
U.S. patent application Ser. No. 289,804 filed on Dec. 27, 1988 (Japanese patent application 62-336121) now U.S. Pat. No. 4,982,271 relates to a motion-adaptive device for separating a luminance signal and a color signal which is designed to resolve the previously-mentioned problems in the adaptive comb filter of Japanese application 60-165889 and the adaptive device of U.S. Pat. No. 4,907,073. The motion-adaptive device of U.S. patent application Ser. No. 289,804 includes a variable vertical-direction filter, a variable horizontal-direction filter, and a time-direction filter composing a three-dimensional spatiotemporal filter. The motion-adaptive device of U.S. patent application Ser. No. 289,804 also includes a motion detector. The characteristics of the three-dimensional spatiotemporal filter are controlled in accordance with the spatial shape and the motion of a picture.
The motion-adaptive device of U.S. patent application Ser. No. 289,804 tends to cause crosstalk in the case of a moving picture. In addition, the motion detector of the motion-adaptive device of U.S. patent application Ser. No. 289,804 requires a large-capacity memory.